The present invention relates to a generator of such signals as a local oscillation signal and a carrier wave signal for use in a high-frequency communication apparatus for satellite communications and the like.
A signal generator capable of stably generating a carrier wave or a local oscillation signal of a high frequency is indispensable for a communication apparatus for use in communication systems such as satellite communications, in which communications are made at a frequency band of high frequency such as a microwave. Further, when such a signal generator is to be used on a mobile member such as a vehicle, it is desired that the signal generator be resistive to vibration.
A conventional signal generator, well known in the market, is described hereinafter.
FIG. 4 is a block diagram showing the conventional signal generator.
In FIG. 4, Phase Lock Loop (PLL) 1 is connected to the frequency control terminal of Voltage Controlled Oscillator (VCO.) 2 through loop filter 3. Output signal 22 of VCO. 2 is split into two portions, of which one is delivered to outside and the other is supplied to the feedback terminal of PLL 1. VCO. 2 is capable of changing the oscillation frequency freely in accordance with the voltage applied to the frequency control terminal.
When a power supply to VCO. 2 is turned ON, a signal of a frequency is generated. The signal generated at this time is unstable and its frequency varies at all times. To stabilize the frequency, a channel setting signal is applied from outside so that the frequency is fixed at a certain level. When the externally applied channel setting signal is fed to PLL 1, PLL 1 varies its output voltage 12 to loop filter 3 thereby fixing the frequency at the certain level. Loop filter 3 smoothes the variation of this voltage 12 and outputs frequency-control-voltage 32 of VCO. 2. VCO. 2 oscillates a signal of the frequency corresponding to voltage 32 applied to the frequency control terminal, and returns the signal to the feedback terminal of PLL 1. PLL 1, again, varies output voltage 12 so as to regulate the frequency. By repeating the operation discussed above, a stabilized signal fixed at a desired frequency can be generated.
In the conventional signal generator as described above, however, the VCO. and the loop filter show great variation in characteristics due to vibration.
Because the frequency of the VCO.-generating signal is vulnerable to the vibration, the feedback operation by means of the above loop construction cannot follow the frequency-changes caused by the vibration.
Accordingly, the signal generator is subjected to great variations in output frequency due to vibration. Therefore, when a signal generator as described above is used in a system of a mobile member such as a vehicle, the system has sometimes required a component such as a damper.
It is an object of the invention to provide a signal generator capable of generating a high frequency for use in satellite communications and the like and free from being affected by vibration.
The signal generator of the invention is constructed as described below.
A digital synthesizer generates a digital signal representative of data corresponding to a desired frequency by means of a logical circuit. A D/A converter converts the digital signal into an analog signal. A bandpass filter eliminates unwanted wave signal components from the analog signal to provide a Radio Frequency (RF) signal. A mixer mixes the RF signal with a local oscillation signal, obtained by frequency-multiplying a reference signal generated from a reference signal generator, to frequency-convert the signals into a high-frequency signal. An output filter eliminates out-of-band noises from the high-frequency signal. The signal generator of the invention delivers the output signal of the output filter.
By the construction as described above, a signal generator capable of generating a high-frequency, without being affected by vibration, can be embodied.
The output signal of this signal generator is useful as a local oscillation signal and a carrier signal in a communication apparatus on a mobile member or the like for use in satellite communications.